The traditional flying adder architecture generates periodic signals with an average frequency output relative to a reference clock by selecting the phases of a set of reference clocks. In order to keep the number of reference clocks reasonably small, the clock phase selection is truncated. The truncation results in consecutive output periods that are not necessarily equal, but that result in a desired average output frequency. Although the desired average output frequency is achieved, the unequal period lengths result in undesired spurious outputs. There is therefore a need for a low power square wave reference without undesired spurious outputs.
More particularly, it will be noted that flying adder frequency synthesizers generate an output pulse train having a frequency but average it over time so that the falling and rising edges of the clocks would not necessarily occur at the same exact period. This results in time jitter when the clock edges are moving around. There are a considerable number of applications where this is acceptable. However, in other applications one requires the rising and falling edges of the clock to occur at precise times so that the clock edges happen at the exact time they are supposed to happen.
Time jitter in prior art flying adder frequency synthesizers is unacceptable in certain electronic warfare applications. For instance, for those systems involving jamming it is very important that the detecting system detects what they perceive to be their own signal returning to them. If the returned signal does not look like their own transmitted signal, the jamming signal will be ineffective because it can be ignored. Thus, there is a requirement for phase coherent signal generation for jamming signals so that the pulses in the jamming signals are edge to edge exact. These type of phase coherent signals are also useful for instance in phased array radars.
An architecture for generating almost-periodic digital signals of a desired average frequency based on a frequency reference clock is known in the art. As is usual, the circuit is driven by a family of uniformly phase shifted copies of a periodic square wave that can be generated by ring oscillator. While the flying adder results in a simple compact fully digital implementation offering good resolution, it also suffers from highly spurious output content due to phase truncation and phase jitter.
Because of its wide tuning range and instant response time, the prior art flying frequency adder is highly suitable for many system on-chip applications. The frequency is controlled by a frequency control word that can be an integer or a fractional number when high frequency resolution is desired. When the frequency control word is an integer, the flying frequency adder can be viewed as a phase divider which can achieve finer resolution than traditional frequency divider circuits are capable of. However, the frequency modulation involved in these flying adder synthesizers results in spurious spikes in the frequency spectrum. Although dithering methods can be used to eliminate or effectively reduce spurs, this approach comes at the cost of increased overall noise.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.